(i) Field of the Invention
This invention relates generally to methods for filling voids with particulate or aggregate material such as crushed rock or gravel, and, more particularly, to an improved method for filling below- and above-ground voids using tailings and similar aggregate materials produced in mining and similar operations.
(ii) Related Art
A number of mining, excavation and construction operations require the deposition of large amounts of aggregate material in some form of cavity or void.
For example, mining operations ordinarily involve removal of ore-bearing rock or earth from a geological formation, thus creating one or more voids in the formation. The excavated rock is typically crushed and processed to extract the ore, leaving the crushed rock residue as tailings. If left unconfined above ground, the massive amounts of tailings produced by a typical mining operation present serious space and environmental problems. However, the tailings are commonly returned into the mine to backfill the voids which have been formed in the underground formation, while in other operations they are deposited in an above-ground pit or holding area.
For example, it is common to mine underground by forming a vertical lift shaft or helical tunnel and then to mine horizontally extending tunnels at different levels. Large volumes of ore are then removed via the horizontal tunnels by blasting a succession of stopes or underground voids upwardly from the far end of each tunnel back toward the center axis. In order for safe mining to proceed it is necessary to backfill each underground void or stope formed as part of the blasting and ore evacuation procedure, so as to support the "roof" above the stope and thereby allow an immediately adjoining volume of ore to be blasted without danger of collapse.
The backfilling is typically carried out by mixing a suitable particulate solid material, usually the mine tailings, with cement and water, and then conveying, trucking or pumping the backfill mixture to the location of the void. Excess water draining from the backfill mixture must be pumped from the mine and the backfill mixture allowed to set to form a solid fill in the stope.
The cost of backfilling is significant and can be as much as 20% of the total cost of the mining operation. The cost of backfilling is directly related to the cost of the cement content in the filling mixture but a significant cost is also involved in transport of the material to the void. The most convenient way of transporting the material to the void is by pumping through pipes but this requires a high water content in the backfill mixture. This is particularly the case with many mine tailings, due in part to the high void content which results from the comparatively uniform aggregate size of the crushed rock.
A conflicting requirement is that, in order to avoid mud slides underground (i.e., the fluidic collapse of part of the backfilled material) the recommended percentage of solids in the fill is above 74%. It is often difficult to pump such a mixture (at this ratio of solids) significant distances, but any increase in water content to improve pumpability increases the risk of mud slides and increases the volume and expense of cement required in the mix to in order for this to cure and reach specified strengths, which are typically in the order of 1 MPa. The amount of cement varies according to the backfill material and the water content but is usually around 6% in order for the fill to reach the required strength.
In some types of mining operations, the tailings are not used to backfill the mine as described above, but are instead deposited above-ground in a large pit or similar containment area. The process is essentially similar to that described above, except that in the absence of a requirement for structural strength the cement or other binder component may be eliminated. The large amounts of water which are required in order to pump the material continue to present serious problems, however, even in an above-ground placement. For example, dams, retaining walls or similar structures must often be provided to prevent mudslides and spills. Moreover, the water often becomes highly contaminated from contact with the tailings (whether from naturally-occurring minerals or from chemicals used in the ore extraction process), with the result that the use of large volumes of water to place the tailings leads to a serious problem containing and treating the water itself.
It is therefor desirable to be able to provide an aggregate fill material which is easy to pump and therefore economic to place, without requiring the high water content which increases the risk of mud slides or which requires a high cement content, which in turn increases the cost of the operation. Such filling material is desirable for use in a wide variety of mining situations, such as, for example, stope fill, the filling-in of disused mines to remove hazardous threats from cave-ins in subsequent open cast mining operations, and other similar situations. Moreover, such filling material is desirable for use in a wide range of other mining and non-mining operations, both above and below ground.